1. Are you using the Yahoo Mail app?
2. Are you using the native Mail app?
3. Do you have email to retrieve via your Yahoo email account?
4. If you've configured Yahoo on your iPhone 5C, go to Settings - Mail, Contacts, Calendars - Yahoo, and make sure that the toggle button is active next to "Mail".
5. Go to Settings - Mail, Contact, Calendar - Fetch New Data - Yahoo, and then make sure that either "Push" or "Fetch" is selected.
6. You can also try either rebooting your iPhone 5C or go to Settings - General - Reset and then select "Reset Network Settings".